Resumen: All communities are dominated by a few species that account for most of the biomass and carbon cycling. On the other hand, a large number of species are represented by only a few individuals. In the case of bacteria, these rare species were until recently invisible. Due to their low numbers, conventional molecular techniques could not retrieve them. Isolation in pure culture was the only way to identify some of them, but current culturing techniques are unable to isolate most of the bacteria in nature. The recent development of fast and cheap high throughput sequencing has begun to allow access to the rare species. In the case of bacteria, exploring this “rare biosphere” has several points of interest. First, it will eventually produce a reasonable estimate of the total number of bacterial taxa in the oceans. Right now we do not even know the right order of magnitude. Second, it will solve the question of whether everything is everywhere or not. Third, it will require hypothesizing and testing the ecological mechanisms that allow subsistence of many species in low numbers. And fourth, it will open an avenue of research into the immense reserve of genes with potential applications hidden in the rare biosphere.